I've entered 50-60 or even more times by choice, on ramdom city locations on the map (encountering or not encountering anything or anyone), and I STILL couldn't find ANY clickable cars. .I've found about 6-or 8 so far I think, but now it's seem like they've all just disappeared, wtf?

Just curios, what are the mathmaetics, the odds, the formulas used to decide when and how easy or often cars can be found? Are the odds greater on clearly city maps, or in those black, darkened city zones (where you STILL find building ruins by entering the world map? Do you have to run into somebody or something, or do you just find'em by choosing to randomply where to search?